Two-component lacquer powders based on masked isocyanates and hydroxyl group-containing polymers having softening points of at least about 40.degree. C. are known in the art. .epsilon.-caprolactam-masked adducts of polyhydric alcohols and polyisocyanates mixed with hydroxyl group-containing polymers are also known (U.S. Pat. No. 3,893,978 corresponding to German Offenlegungsschrift No. 2,215,080).
It has now surprisingly been found that polyisocyanates which have isocyanurate groups and in which the free isocyanate groups are masked with lactam may be used as mixtures with a polyhydroxyl component and optionally the conventional auxiliary agents and additives to serve as new two-component polyurethane lacquer powders which are resistant to yellowing.
Two-component polyurethane lacquer powders based on a mixture of polyisocyanates which have isocyanurate groups and masked isocyanate groups and a polyhydroxyl compound have not hitherto been known.
These new two-component lacquer powders are particularly distinguished from all previously known polyurethane lacquer powders by the following features:
(1) Polyurethane coatings which are resistant to yellowing may even be produced from aromatic diisocyanates which tend to undergo yellowing, and they are also distinguished by advantageous mechanical properties, chemical resistance, good adherence, gloss and good resistance to weathering thereof; PA1 (2) Cross-linking with the polyhydroxyl component at an isocyanate/OH ratio of only about 0.5:1 is sufficient to give rise to excellent polyurethane coatings. PA1 (a) an isocyanate component having masked isocyanate groups; PA1 (b) a polyhydroxyl component; and, optionally, PA1 (c) the conventional auxiliary agents and additives; PA1 X represents a nitrogen atom, in which case
Both the possibility of using aromatic diisocyanates for preparing the products according to the present invention, and the possibility of obtaining optimum film properties with the above-mentioned "sub-cross-linking" clearly show that the present invention provides an exceptionally economical process for coating any substrates.